The magic beasts with low levels of intellect didn't care about the mist at all and they continue breathing as usual causing them to inhale the mist. The potion was after all not colorless and odorless. A creature with a slightly higher intellect might raise their guards against the mist but these magic beasts didn't. They simply let the mist enter their system and it showed its effects
The eyes of the magic beasts soon turned red, a huge sense of irritation and bloodthirst bloomed inside them. Their emotions welled up and in time, they started attacking each other
While still living in the forest, Rezen already used something similar to the potion he just used that causes the magic beasts to fight against each other. The potion he used now was an upgraded version of that and is a genuine alchemical potion
